The invention concerns a work unit (or operating unit) for automatic machine tools, also known as transfer machines.
As is already known, these machines make it possible to carry out a series of successive machining operations (drilling, screw-threading, etc.) on the same workpiece, in very short times by virtue of the maximum reduction of the idle times between one machining operation and the other.
This occurs because the workpieces are mounted on a rotating table round which there are located work units, variable in number depending on the number of single operations, and which execute predetermined operating cycles while the workpieces are brought in front of them by the controlled rotations of the table; the latter may have a horizontal or vertical axis, depending on the model of machine.
The present invention relates to a new type of machining unit.
Said units comprise a spindle intended to actuate a tool, which is supported in such a manner as to have in addition to the normal rotational work movement about its axis (commonly indicated as the "Z" axis), also that of advance and return along the said axis.
In order to obtain these effects it is now an established practice to mount the spindle rotatably inside a sleeve where, however, it is locked axially, causing the said sleeve instead to be able to slide along the work axis of the spindle; the desired axial translational movements of the latter are therefore obtained by causing it to move integrally with the sleeve and independently of its speed of rotation.
To this end, for the movement of the spindle/sleeve assembly, various alternative solutions are known which can be grouped principally into electromechanical and hydraulic solutions; the first indeed make use of mechanisms of various types (worm gears with ball recirculation or the like) driven by electric motors, while the second use a pressurized fluid to displace the sleeve like a doubleacting piston.
Both of these categories of axial control of the spindle/sleeve assembly, however, render their development problematical, especially in view of obtaining better performances with regard to the axial velocity and acceleration of the spindle.
It is in fact necessary to take into account that an increase in these parameters has immediate repercussions on the reduction of the idle times during machining and therefore on the productivity of the entire machine, thus making this of particular interest.
However, with the current electromechanical or hydraulic systems a significant increase in the velocity and acceleration of the spindle along its axis entails an increase in the complexity and dimensions of the corresponding work unit, such as to render this eventuality not very convenient.